This invention relates generally to control circuits for use in combination with electric power systems, and particularly, to such control circuits which respond to a difference in current flow at different locations along a power conductor.
Constant speed drive electric power systems which are typically found on commercial aircraft, couple an electric generator to the aircraft engine through a hydromechanical transmission which drives the generator at a constant speed to produce a constant frequency output voltage. Variable speed constant frequency (VSCF) power systems include a generator which is directly coupled to the engine and therefore driven at variable speeds. The variable frequency output of this generator is electronically converted to a constant frequency output. It is desirable to retrofit existing constant speed drive equipped aircraft with variable speed constant frequency power systems. A key to successful retrofit lies in the design of a VSCF system which is directly interchangeable with the existing constant speed drive system. This precludes aircraft wiring changes or changes in any of the other system components. To accomplish this objective, the VSCF converter and its controls must be in the same package as the generator and a remote generator control unit is positioned closer to the system loads.
Differential protection circuits which monitor current at different locations in a power system and produce a control signal for de-exciting the voltage generating source when the difference in currents exceeds some predetermined value, are well known in the art. Typical differential protection circuits contain two current transformers which respond to electric current in a power conductor and are connected in a loop. At least one burden resistor is connected across the current transformers and the polarity of the voltage developed across the transformers is such that if the same current flows through both transformers, no voltage is developed across the burden resistor. If a fault occurs on the power conductor between the two transformers, a voltage is developed across the burden resistor. A control circuit detects the presence of this voltage and takes appropriate action by, for example, disabling the power source or disconnecting the power conductor from the load.
A differential protection circuit for use in a VSCF aircraft power system that is used to replace a constant speed drive system must sense faults at both the VSCF system and the remote generator control unit. This sensing is complicated by the fact that the ground potential near the VSCF system may be different from the ground potential near the remote generator control unit because of common mode voltage in the aircraft structure. In addition, very high voltages can be introduced in the aircraft structure as a result of a lightning strike. It is therefore desirable to devise a differential protection circuit which can function in a VSCF system that is used to replace a constant speed drive system.